AMHS's have been used extensively in the semiconductor fabrication field. The typical system includes a plurality of bays (rows) of storage areas. Each bay has a stocker, which includes bins for holding a plurality of containers, such as standard mechanical interfaces (SMIFs) for loading 200 mm (8 inch) wafers, or front opening unified pods (FOUPs), which may be used to load 300 mm (12 inch) wafers. The stocker holds the SMIFs or FOUPs in preparation for transporting a SMIF or FOUP to the loadport of a processing tool. An overhead hoist transport (OHT) associated with each bay transports the SMIF or FOUP with wafers from the stocker to a loadport for processing in one of the tools (fabrication process machines). The OHT also moves empty vehicles to the loadport to receive processed wafers to remove from the tool.
AMHS's suitable for use in fabricating 200 mm wafers are in a mature state, whereas AMHS systems for use in 300 mm wafer fabrication include many new design features. An important factor in designing the newer AMHS systems is efficiency. Key indices are defined to monitor the efficiency of the AMHS, and that efficiency has a direct impact on the operation of the fabrication facility (fab). The operation service time (OST) has been defined as a key index to monitor AMHS intrabay efficiency by tool loadport. The OST is a measure of how long an operator must wait to receive a lot of wafers (for processing) after a retrieval command is sent to the stocker. The AMHS is triggered to pick up a lot of wafers and then, when the lot has completed on the tool loadport, the AMHS inputs the next lot on the loadport.
FIG. 1 shows a conventional AMHS workflow. In a known system, the intrabay operation is as follows. When the lot 22 of wafers is processed, a “remove” command is issued by the loadport of tool 21. The intrabay OHT receives the request, and the AMHS dispatches an empty vehicle 31 to pick up the processed lot 22, as shown by arrow 11. The lot 22 is transferred to the empty vehicle 31, in a step indicated by arrow 12. This time period from when the loadport issues a “remove” command to when the lot 22 is picked up from the tool 21 is referred to as the OST-out-time. Next, the empty loadport issues a “move-in” command to request the next lot 24. The intrabay OHT delivers a next lot 24 to the empty tool loadport, shown by arrows 13–15. The stocker 23 transfers the next lot 24 to a vehicle 32, in a step indicated by arrow 13. The vehicle 32 moves to the location of the tool 21, in a step indicated by arrow 14. The lot 24 is input, as shown by arrow 15. The time for this operation is called the OST-in-time. In a typical system, the total cycle time for this operation is the OST-out-time plus the OST-in-time. For example, if the OST-out-time is about 1 minute and the OST-in-time is about 3.5 minutes, the total time to move the next lot to the loadport is 4.5 minutes.
It would be desirable to reduce the transfer time required by the AMHS.